(1) Field of the Invention
This invention relates to a permanent magnet material of a bulk shape and, in particular, to a rare earth metal-iron-boron (R-Fe-B) permanent magnet material with a high energy product.
(2) Description of the Prior Art
Permanent magnets have been used in various applications such as electromechanical apparatus.
Recently, demands for Sm-Co permanent magnets have increased in place of known alnico magnets, ferrite magnets, and other conventional magnets, because of the high energy product of Sm-Co magnets. However, the Sm-Co magnets are expensive because of use of cobalt.
Therefore, various approaches are made for new permanent magnets which are economical and have an increased energy product.
A possible approach has been directed to a novel intermetallic compound of transition metal (T) and rare earth metal (R) instead of the Sm-Co intermetallic compound.
However, the intermetallic compounds without use of Co have been considered impossible to produce a magnet having coercivity which is associated with magnetocrystalline anisotropy because the compounds have an easy magnetization direction in the crystal phase. A reference is made to K. J. Strnat; IEEE Trans. Mag. (1972) 511.
In Appl. Phys. Lett. 39(10) (1981), 840, N. C. Koon and B. N. Das disclosed magnetic properties of amorphous and crystallized alloy of (Fe.sub.0.82 B.sub.0.18).sub.0.9 Tb.sub.0.05 La.sub.0.05. They wrote that crystallization of the alloy occurred near the relatively high temperature of 900 K, which also marked the onset of dramatic increase in the intrinsic coercive force. They found out that the alloy in the crystallized state appeared potentially useful as low cobalt permanent magnets.
It is considered that magnetically hard intermetallic compound of R-Fe-B (R=Tb and La) is formed in the alloy. Reviewing the R-Fe-B (R=Gd, Sn, Nd), ternary phase diagram by N. F. Chaban, Y. B. Kuz'ma, N. S. Bilonizhko, O. O. Kachmar and N. W. petriv; Dopodivi Akad. Nuk. Ukr. RSR, Ser. A (1979) No.10, P.P. 875-877, the intermetallic compound R-Fe-B (R=Tb and La) by Koon et al is guessed to be represented by R.sub.3 Fe.sub.16 B, which is confirmed to be Nd.sub.2 Fe.sub.14 by J. J. Croat et al. Reference is made to J. J. Croat, J. F. Herbst, R. W. Lee and F. E. Pinkerton; J. Appl. Phys, 55 (1984) 2078.
Therefore, considering the saturation magnetization of an intermetallic compound of R-T as shown in the above-described reference by K. J. Strnat, it can be guessed that use of Ce, Pr, and/or Nd for R in Fe-B-R alloy can provide better magnetic properties for permanent magnets than the Fe-B-La-Tb alloy.
J. J. Croat proposed amorphous (Nd and/or Pr)-Fe-B alloy having magnetic properties for a permanent magnet as disclosed in JP-A-60009852. Those magnetic properties were considered to be caused by a microstructure where Nd.sub.2 Fe.sub.14 B particles having a particle size of 20-30 nm were dispersed within an amorphous Fe phase. Reference is further made to R. K. Mishra: J. Magnetism and Magnetic Materials 54-57 (1986) 450.
However, the amorphous alloy can provide only an isotropic magnet because of its crystallographically isotropy. This means that a high performance permanent magnet cannot be obtained from the amorphous alloy.
Sagawa, Fujiwara, and Matsuura proposed an anisotropic R-Fe-B sintered magnet in JP-A-59046008 which was produced from an ingot of an alloy of R (especially Nd), Fe, and B by conventional powder metallurgical processes. The sintered magnet has more excellent magnetic properties for permanent magnets than the known Sm-Co magnets.
The R-Fe-B sintered magnet comprises a metallic solid solution phase and magnetic crystalline particles dispersed within the metallic solid solution. Each the magnetic crystalline particles comprises an intermetallic chemical compound represented by R.sub.2 Fe.sub.14 B. The metallic solid solution phase comprises the R rich alloy out of stoichiometric compound of R.sub.2 Fe.sub.14 B. Since R especially Nd is active to oxygen and the R rich solid solution phase is very active to oxygen. Therefore, any care is necessary so as to prevent the magnet from oxidation.
In production of the R-Fe-B sintered magnet, an R rich ingot of the R-Fe-B alloy is prepared and is pulverized and ground into a powder having an average particle size of about 3-5 .mu.m. The powder is compacted into a desired shape and is sintered. However, the ingot comprises the magnetic crystalline phase of the chemical compound R.sub.2 Fe.sub.14 B and the solid solution phase. Therefore, the alloy tends to be oxidized in production of the magnet, especially at the grinding step. Actually, the sintered R-Fe-B magnet usually contains oxygen of about 3,000 ppm.
Furthermore, the solid solution phase can hardly be finely ground and the ground powder unavoidably contains coarse particles of the solid solution phase in comparison with the R.sub.2 Fe.sub.14 B particles after the grinding step. Therefore, it is impossible to uniformly mix the solid solution powder with the R.sub.2 Fe.sub.14 B powder. This means that magnetic particles are not uniformly dispersed in the solid solution phase in the sintered magnet, which impedes enhancement of the magnetic properties.
It is desired for obtaining a high energy product that the amount of the solid solution phase be reduced. However, decrease of amount of the solid solution phase results in incomplete sintering.